QUESTION 1: STATE POINT ANALYSIS OF SECONDARY CLARIFIER

We have a plant with biological nutrient removal and secondary clarification. The design influent flow rate of the plant is 120,000m³/d and the plant will utilize 8 secondary clarifiers each with an area of 350 m². We wish to use a low recycle ratio of 0.4 and the bioreactor will have a total solids concentration of 4,000 mg/L.

NOTE: A revised slide for secondary point analysis has been uploaded to LMS. Please refer to this slide before beginning the question. As per usual it is advised to do this in Excel. Note that X in the solids design equations refers to total sludge, i.e. it is the equivalent of XT in our bioreactor equations.

Part A: Plot the total solids flux and determine whether the recycle rate is below, at, or above the critical recycle ratio.

Part B: Given the following settling velocity data i) use state point analysis to determine whether the suggested design configuration is suitable and if not what is limiting the design. ii) Propose two possible remedies without changing the clarifier design.iii) Why would we prefer to change the clarifier area than either of these two options?

Settling velocity data

Part C: Assume that all design parameters above are fixed with the exception of clarifier area as all other elements are associated with our biological tank design. Propose a suitable clarifier area to the nearest 25 m². Do this question visually rather than as an exact calculation.

QUESTION 2: DISINFECTION

Our treatment process is very simple and consists of fine screens, a microfiltration membrane bioreactor and then chlorine disinfection for a treatment plant with an inflow of 50,000m³/d. Our regulatory requirement states that we must achieve an8 log reduction of total coliforms and 5 log reduction of Cryptosporidium (protozoa).

Part A: Determine the log reduction required by chlorination for total coliforms and Cryptosporidiumif our fine screens provide (essentially) zero pathogen removal and our MBR provides 4 log removal for both bacteria and protozoa.

Part B: If our effluent from the MBR contains NH3 at a concentration of 0.4 mg-N/L what is the required chlorine dose to meet the break point?

Part C: Determine the required overall chlorine concentration required achieve our disinfection goals for a contact basin with a volume of 3125 m3. Aim to provide enough free chlorine sufficient for the disinfection, i.e. ignore the disinfection performed by combined chlorine. Use the tables provided in the lecture notes and assume the pH is low enough that all free chlorine is in the form HOCl. Use the upper values of reported dose to ensure we achieve our target.